554 Prof. K. T. Compton on Ionization and Production 



an electron may displace an electron from a more to a less 

 stable state ; whereas, if an electron falls directly from a 

 less to a more stable state, ii emits radiation whose frequency 

 equals the difference of energy in the two states divided by h. 

 Bohr's theory of atomic structure is based on these consi- 

 derations, and, together with its modifications by Debye and 

 Sommerfeldt, has been remarkably successful in accounting 

 quantitatively for the spectra of systems constituted of a 

 single electron and nucleus. The theory has not been 

 developed to account quantitatively for the spectra of more 

 complicated systems, although, if certain assumptions are 

 made, the convergence frequencies may be calculated in 

 some cases. It is obviously of the greatest importance to 

 secure definite information regarding the spectra and energies 

 of formation of those atoms (other than atoms with a single 

 electron) for which the theoretical assumptions may be most 

 easily put into quantitative form. Of these cases, the simplest 

 is the normal helium atom, with its single nucleus and two 

 outer electrons. 



In Bohr's model of the normal helium atom the two electrons 

 are symmetrically located in the same orbit, and their com- 

 bined energy is equal to e x 83 volts/300. If one electron is 

 removed, and the remaining electron takes its most stable 

 position, the energy equals e x 54*3 volts/300. The difference 

 gives 28*7 volts as the ionizing potential ; and the corre- 

 sponding frequency gives 430 A as the convergence wave- 

 length of the fundamental series. The frequencies of the 

 other lines in the series cannot be calculated without a 

 knowledge of the behaviour of one electron while the other 

 is being displaced from orbit to orbit. Analogy with hydrogen 

 would suggest that 3/4 of 28'7, or 21*5 volts should be 

 the resonance potential of helium. There is no reason, 

 however, for believing that hydrogen and helium should have 

 homologously spaced series. Evidently quite different results 

 from those obtained by Bohr might be obtained by assuming 

 a different configuration of electrons in the normal atom*. 



Early experimental determinations of the minimum ionizing 

 potential of helium | indicated a value between 20 and 21 volts. 

 Recent discoveries, stimulated by applications of Bohr's 

 theory, suggest that this is really a resonance potential, the 

 effects previously attributed to ionization being accounted 



* Lande, Phys. Zeit. xx. p. 228 (1919). 



f Franck and Hertz, Verh. d. D. Phys. Ges. xv. p. 34 (1913) ; Fawlow, 

 Roy. Soc. Proc. A. xc. p. 398 (1914) ; Bazzoni 7 Phil. Mag. xxxii. p. 566 

 (1916). 



